Radio audio signals, whether frequency modulated (FM) stereo or amplitude modulated (AM), are often plagued with background noise caused by, among other things, disruption of the radio signal or electromagnetic interference from external sources. In an automotive FM/AM receiver, the environment is rife with sources of electromagnetic signals that fall within both the AM and the FM band.
One source of these signals is the ignition system of the vehicle. The electric spark used to ignite the fuel/air mixture, as well as the current flowing in the cables from the power source to the spark plugs, generate electrical interference which is picked up by the vehicle's FM/AM radio receiver. Interference generated by the ignition system is commonly categorized as impulse noise and causes a “popping” sound from the automotive speakers. In addition to the ignition impulse noise, automotive radio receivers are susceptible to interference created by various electrical motors of the vehicle, such as power window DC motors. In the typical case, this noise is a nuisance, but greater interference can render the radio unusable.
Traditionally, automotive FM/AM receivers have been provided with weak signal processing handles or functions, such as stereo flat blend, stereo high blend, high-frequency roll-off and audio attenuation. Tile general purpose of these functions is to reduce noise and distortion caused by multipath fading, adjacent channels and momentary loss of signal. The controls for these functions are typically derived from the Received Signal Strength Index (RSSI), ultra-sonic noise (USN), noise within the demodulated composite signal above all audio information, and wide-band amplitude modulation (WBAM). An audio processor block accepts these controls and determines the correct amount of audio processing functions to apply.
Among the weak signal processing functions, audio attenuation is, as the name implies, a gain stage controlled by the audio processor that acts upon the left and right audio channels. This is typically used at very weak signal levels after all other audio processing functions have been used. Audio attenuation is typically a last resort to quiet the noise by quieting all of the audible frequencies.
High-frequency roll-off, or high-cut, utilizes a low-pass filter with a corner frequency controlled by the audio processor. When activated, high-cut deliberately limits the bandwidth of the audio signal to attenuate noise in the high frequency range of the audio spectrum. High-cut weak signal processing is applied to the left and right audio paths.
Flat blend is the attenuation of the L-R stereo portion of the received signal. Because the noise spectrum of the demodulated FM signal increases as the square of the frequency, the noise content in the stereo portion of the signal is much greater than the mono. Therefore, blending to mono is advantageous when there is a significant amount of noise. Theoretically, a 26 dB improvement can be obtained with the flat blend entirely at mono. A substantial reduction in noise can still be attained by blending to less than full mono, leaving some stereo audio component.
High-blend weak signal processing is essentially high-cut applied on the stereo L-R path. The goal of high-blend is to have the same effect on the stereo noise as flat blend, but maintain stereo separation at lower frequencies. Since stereo separation is not very perceivable at higher frequencies, using high-frequency roll-off on the stereo path will reduce noise with little noticeable loss in stereo separation.
In order to account for impulse noise, many audio processors include a noise blanker. The concept behind the noise blanker is to detect the impulse and then appropriately blank the audio in relation to the disturbance. In one approach, the received signal is blanked in timing with the firing of the engine spark plugs, as described in U.S. Pat. No. 5,890,059, which disclosure is incorporated herein by reference. Another approach is a blanking circuit 10 depicted in FIG. 1. In this approach, the composite signal 11 is fed through two paths. In the first path, the signal passes through a delay 12 that delays passage of the signal for a pre-determined length of time calibrated to the passage of the signal through the other path. Following the delay 12, the incoming composite signal is fed to a signal hold section 13.
In the second path, the incoming signal 11 passes through a high pass filter 15 that filters off all of the composite signal (FM or AM), leaving the ultra-sonic noise (USN). The filtered signal is fed to a comparator 16 which compares the USN to a threshold value 17. If the USN signal from the high pass filter 15 exceeds the threshold value, then the output of the comparator 16 goes high and a blanking pulse 18 is created. This blanking pulse 18 is fed to the signal hold section 13 which holds the signal fed to the output 14 at the value of the signal immediately before the impulse occurred. The delay 12 is calibrated to compensate for the delay as the signal 11 passes through the filter 15 and comparator 16.
The blanking circuit 10 in FIG. 1 can have many forms, such as a simple hold or a linear interpolation. However configured, the circuit holds the output signal 14 at something that better represents what the composite signal should be in the absence of the impulse noise. However, blanking circuits of this type are susceptible to distortion in the audio, especially for impulse rates of 1 kHz or higher, which typically corresponds to DC motor noise. Thus, there remains a need for an improved blanking approach that eliminates this unwanted distortion, while also eliminating the unwanted impulse noise.